The present invention relates to a hot anode plasma torch having a gas shroud heated by the anode for providing greater stability of the plasma effluent jet, while maintaining high core temperatures and velocities, to provide a favorable environment for heating of particles injected into the plasma jet.
Conventional plasma spraying in atmospheric air suffers from severe air entrainment into the plasma jet. This air entrainment entails two major problems: (1) it cools the plasma jet, leading to a rapid decay of the jet temperature and the associated heating of particulates injected into the plasma jet during the spray process; (2) the entrained air causes oxidation of metallic particulates which frequently precludes utilization of this approach for spraying of metals and alloys. In order to prevent this problem, low pressure plasma spraying (LPPS) has been introduced where the spray process is done in an environmental chamber at reduced pressures and in a controlled atmosphere (for example in argon).
Although this process produces excellent coatings, it is extremely expensive. Even though the aircraft industry has been using this process extensively for spraying of engine parts efforts are being made to move away from this expensive technology and gas shrouding of the plasma jet is one of the approaches which is under consideration. This process will at least delay the entrainment of air during atmospheric pressure plasma spraying. In the prior art, various plasma torches have been utilized which have a gas shroud for the torch. Such a device is shown in U.S. Pat. No. 3,470,347 which shields the gas effluent with a ring of fluid flow. However, this particular torch does not include a hot anode and hot shroud, which is desirable from the standpoint of obtaining high performance plasma, and merely has an annular jet or flow of gases that surround the plasma core.
U.S. Pat. No. 4,121,082 shows a structure that has a preheated gas shield that directs a flow of gas back toward the plasma source, that is, back toward the anode and cathode, so that the shroud gas flow is a reverse direction from the plasma effluent.
U.S. Pat. No. 4,841,114 also showns a high velocity, control temperature plasma spray method having an elongated anode that has a gas stream on the interior of the anode to surround the core of plasma.
In an article entitled "A Gas-Shrouded Plasma Spray Torch" by Fleck et al, and published in the Proceeding of the 7th International Symposium on Plasma Chemistry, Vol. 4, page 1113 (1985), a description of a gas shroud using a conventional torch indicated that the plasma effluent increased in length, and through modeling described in the paper, there was a predicted reduction of oxygen entrainment and consequently the reduction of oxide formation in the coating. Thus, the shrouding of a plasma torch reduces the oxides that can appear in the coating that is being formed and is desirable.
The use of a hot anode, that is, an anode that is not cooled with water or other liquid provides a way of having a shroud gas flow that is heated by the anode, and thus aids in cooling the anode, the shroud gas flow surrounds the anode, and then intermixes with the plasma as the plasma flows out of the plasma torch.